Octahedral 5d ~ 7-ion Spin Hamiltonian Parameters Of Theoretical Research

Transition-metal ions usually act as important active centers in various functional materials. The properties of these materials largely depend upon the local structures of the dopants. Electron paramagnetic resonance (EPR) is a useful tool to investigate local structures and electronic states of paramagnetic ions in crystals, and the EPR experimental results are normally described by the spin Hamiltonian parameters, i.e., g factors, hyperfine structure constants and superhyperfine parameters. Since important information about defect structures and spin levels can be obtained by analyzing the spin Hamiltonian parameters and their microscopic mechanisms, theoretical studies on the EPR spectra and defect structures for transition-metal ions in these materials are of scientific and practical significance.5d7 ions belong to the important systems of transition-metal groups, which exhibit unique catalytic, luminescence and photographic properties in crystals and promise bright prospects for these materials. To investigate the relationships between local structures and functions of these systems, EPR experiments have been carried out for MgO:Pt3+, AgBr:Ir2+ and AgCl:Ir2+ crystals, and the spin Hamiltonian parameters were measured for the tetragonal Pt3+ and Ir2+ centers in MgO and AgBr and the tetragonal and orthorhombic Ir2+ centers in AgCl. Up to now, however, the theoretical analysis for the above experimental results has been unsatisfactorily performed. First, the previous studies on the g factors for 5d7 ions were normally based on the perturbation formulas from the conventional crystal-field model, while the contributions from the ligand orbital and spin-orbit coupling interactions were not taken into account. In fact, these treatments seem unsuitable for a 5d7 ion in crystals due to strong covalency. Second, the EPR analysis failed to connect with the local structures of the systems. For example, the contributions from the low symmetrical distortions (e.g., the axial elongation due to the Jahn-Teller effect and the ligand displacement due to the charge compensator) were not quantitatively included in the calculations. Third, in the previous studies of the superhyperfine parameters, the ligand unpaired spin densities were not theoretically determined in a uniform scheme but obtained by fitting the experimental superhyperfine parameters. Because of the above imperfections, the previous results were usually not in good agreement with the experimental data, and the information about local structures was not acquired for these impurity centers yet.In order to overcome the shortcomings of the previous works and to investigate the EPR spectra and local structures of these 5d7 centers more exactly, in this work, the improved perturbation formulas of the spin Hamiltonian parameters for 5d7 ions under tetragonally and orthorhombically elongated octahedra are established based on the cluster approach. In these formulas, the ligand orbital and spin-orbit coupling contributions are taken into account, and the related parameters (e.g., tetragonal or orthorhombic field parameters, the molecular orbital coefficients and the unpaired spin densities) are correlated to the local structures and optical data of the systems. Thus, the EPR analysis is correlated to the local structures of the impurity centers. These formulas are applied to the tetragonal Ir2+ (and Pt3+) centers in AgBr (and MgO) and the tetragonal and orthorhombic Ir2+ centers in AgCl. The EPR spectra are satisfactorily explained, and the information about defect structures is obtained for these centers. As for the tetragonal Pt3+ (or Ir2+) center in MgO (or AgBr), the substitutional impurity on the cation site may suffer the relative elongation by an amountΔZ≈0.08 ? along [001] (or C4) axis due to the Jahn-Teller effect, which lowers the local symmetry from the original cubic (Oh) to tetragonal (D4h). As regards AgCl:Ir2+, there are two defect centers at different temperatures. At high temperature, Ir2+ in AgCl exhibits the tetragonally elongated center similar to that in AgBr:Ir2+ due to the Jahn-Teller effect. At low temperature, the orthorhombic Ir2+ center is produced due to the next nearest neighbour silver vacancy VAg along [100] (or X) axis. The intervening Cl? in the central Ir2+ and the VAg can experience the inward displacementΔX≈0.004 ? toward the Ir2+ along X axis due to the electrostatic repulsion of the VAg, apart from the Jahn-Teller elongation along [001] (or Z) axis. Based on the calculations, the low covalency factors (≈0.6 ~ 0.7) and the significant orbital admixture coefficients (≈0.5~ 0.8) reveal considerable covalency and impurity-ligand orbital admixtures for all the systems. Therefore, the contributions from the ligand orbitals and spin-orbit coupling interactions are important and should be considered. In addition, the theoretical superhyperfine parameters based on the present cluster approach calculations show reasonable agreement with the experimental data.